multi: don't pass err.Error() to Fatalf

It resulted in interpreting the error message as a format string.
Use Fatal(err) instead.

discovery/syncer_test.go

@@ -473,7 +473,7 @@ func TestGossipSyncerApplyNoHistoricalGossipFilter(t *testing.T) {
 	// filter.
 	err := <-errChan
 	if err != nil {
-		t.Fatalf(err.Error())
+		t.Fatal(err)
 	}
 }